linux-kernel - Re: [PATCH 0/6] Introduce CET supervisor state support

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Message-ID: <73802bff-833c-4233-9a5b-88af0d062c82@intel.com>
Date: Fri, 12 Jul 2024 07:36:39 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "Yang, Weijiang" <weijiang.yang@...el.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "x86@...nel.org" <x86@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>
Cc: "peterz@...radead.org" <peterz@...radead.org>,
 "john.allen@....com" <john.allen@....com>, "Gao, Chao" <chao.gao@...el.com>,
 "mlevitsk@...hat.com" <mlevitsk@...hat.com>
Subject: Re: [PATCH 0/6] Introduce CET supervisor state support

On 7/11/24 15:55, Edgecombe, Rick P wrote:
>> Where current is a 'task_struct' which is in /proc/slabinfo and 'struct
>> thread_struct thread' and 'struct fpu' are embedded in 'task_struct',
>> not allocated on their own:
> I think thread_struct is always a slab, but the current->thread.fpu.fpstate
> pointer can be reallocated to point to a vmalloc in fpstate_realloc(), in the
> case of XFD features.

Good point. I was ignoring XFD and AMX.  They're super rare and
(conditionally) add another 8k. -- Well, closer to 11k since we
duplicate some of the XSAVE area. --  But honestly, even if the AMX
'struct fpu' fit perfectly into 4k*3 pages and CET-S made it take 4k*4
pages,  I'm not sure I'd even care.  It would only affect AMX-using apps
on AMX-capable hardware.  So a small minority of tasks on a small
minority of one x86 vendor's CPUs.

The (potential) space consumption from the inline task_struct fpu will
matter a lot more across all Linux users than AMX ever will.  It would
affect all tasks on all CPUs that have CET-S which will hopefully be the
majority of x86 CPUs running Linux some day.